Does It “Rain” Diamonds on Neptune and Uranus?

Abstract

The methane phase diagram is extremely important to understand the physical properties of the ice giants─Uranus and Neptune. Several experimental studies using diamond anvil cells (DACs) combined with laser heating have reported the formation of diamond from methane at pressures of 10–80 GPa and temperatures above 2000 K, corresponding to the interiors of these planets. These results, however, are probably affected by the presence of metallic heat absorbers, widely used in all previously reported experiments and interacting with hydrogen at high pressure. In the present work, the effect of metallic heat absorbers on the decomposition of methane into diamond was studied at 20–95 GPa and 1300–3700 K using laser-heated DACs with platinum (as hydride-forming) and gold (as non-hydride-forming) metals. In the case of a platinum heat absorber, diamond formation was observed from 50 to 95 GPa near 2000 K simultaneously with platinum hydride formation. In contrast, in the case of a gold heat absorber, diamond formation was not observed below 95 GPa and 3700 K. Thus, the hypothesis of diamond precipitation in the Uranus and Neptune interiors should be reconsidered, taking into account the effect of metallic heater reactivity on the experimentally observed reactions.